Refactor Spec.run to macro finished

This should be largely equivalent to the current solution with `at_exit`
but more robust because it does not conflict with libraries' `at_exit`
handlers.

src/spec.cr

@@ -132,4 +132,9 @@ Spec.add_split_filter ENV["SPEC_SPLIT"]?
   Process.on_interrupt { Spec.abort! }
 {% end %}
 
-Spec.run
+# The spec suite should run as last thing in the program. This enables other
+# top-level code (for example library configuration) to run before the spec
+# suite starts.
+macro finished
+  Spec.run
+end

src/spec/dsl.cr

@@ -205,24 +205,19 @@ module Spec
   def self.run
     @@start_time = Time.monotonic
 
-    at_exit do |status|
-      # Do not run specs if the process is exiting on an error
-      next unless status == 0
-
-      begin
-        if Spec.list_tags?
-          execute_list_tags
-        else
-          execute_examples
-        end
-      rescue ex
-        STDERR.print "Unhandled exception: "
-        ex.inspect_with_backtrace(STDERR)
-        STDERR.flush
-        @@aborted = true
-      ensure
-        finish_run unless Spec.list_tags?
+    begin
+      if Spec.list_tags?
+        execute_list_tags
+      else
+        execute_examples
       end
+    rescue ex
+      STDERR.print "Unhandled exception: "
+      ex.inspect_with_backtrace(STDERR)
+      STDERR.flush
+      @@aborted = true
+    ensure
+      finish_run unless Spec.list_tags?
     end
   end